Did you use storage seal when you winterised the engine? Sounds more like the oily black stuff running down the exhaust (leg) and out of the water pick up and exhaust of the prop. Are you sure its gear oil?

Pretty sure it's gear oil- blue and fresh. Engine had been laid on its side, so I guess gear oil could have flowed to the water intake and only dribbled out when I stood it upright.

Hasn't leaked any more during the week so will will drain and pressurise this weekend - thanks for the idea. If it passes the pressure test then I guess no water is going to get anywhere it shouldn't!